Effective methods for the recovery and/or separation of particular divalent metal ions, such as (a) Ca2+ from source solutions containing Mg2+ and/or other ions and (b) Cd2+ from source solutions containing Zn2+ and/or other ions, are of great importance in modern technology. It is particularly difficult to remove these particular metal ions in the presence of moderate to strong acids and soluble complexing or chelating agents, such as the halide ions, which have a high affinity for the desired metal ions. It is also difficult to remove the aforementioned divalent metal ions when they are present at low concentrations in solutions containing other metal ions at much greater concentrations. Hence, there is a real need for a process to selectively concentrate certain divalent metal ions when present at low concentrations and particularly when in the presence of acid solutions and other complexing agents.
Some methods for the recovery of divalent metal cations from solution are known in the art. However, the removal and/or separation of specific divalent metal cations is often complicated by a myriad of conditions including the presence of other divalent cations, the presence of other cations in very high concentrations, and the presence of other chelating agents. It is also known that ligands present as solutes in a solvent have the ability to selectively form strong bonds with particular ions or groups of ions present as solutes in the same solvent according to size, donor atom-related properties and other known selectivity characteristics.
Ethyleneglycol-bis-(xcex2-aminoethyl ether)-N,N,N,Nxe2x80x2-tetraacetic acid and (ethylenediamine)tetraacetic acid, commonly referred to as EGTA and EDTA respectively, are both ligands that may be used as solutes to complex divalent ions. However, EGTA has a greater selective preference for larger divalent metal ions than EDTA. See Critical Stability Constants, Volume 1: Amino Acids, A. E. Martell and P. M. Smith, Plenum Press, N.Y. and London, 1974. Therefore, EGTA is an important ligand for use in complexing larger divalent metal ions.
This being the case, researchers have not previously been able to incorporate EGTA into phase separation systems. This is significant because EGTA present in solution as a solute simply acts to complex selected ions, but does not provide a means for their separation. Specifically, never before has EGTA been successfully covalently bonded to a membrane support. As such, EGTA bonded membranes have not been used in phase separation processes for removing, separating and concentrating larger ionic radii or desired divalent ions from solutions, particularly where such desired ions are present in solutions with smaller and/or similar ions present at a much higher concentration.
As such, it would be useful to provide a composition and method for removing, separating, and/or concentrating certain desired divalent metal ions in solution from other ions, such as (a) Ca2+ from Mg2+ and/or (b) Cd2+ from Zn2+, even when the desired ion is present at a low concentration.
The present invention is drawn to a novel composition comprising an EGTA ligand covalently bonded to a membrane. The invention is also drawn to methods for removing, separating and/or concentrating certain desired divalent metal ions including (a) Ca2+ from source solutions containing Mg2+ and/or other ions and (b) Cd2+ from source solutions containing Zn2+ and/or other ions. In fact, the removal of these ions (Ca3+ and/or Cd2+) may occur when they are present at from very low to very high concentrations, i.e., from ppb to g/l levels of Cd2+ and/or Ca2+.
The concentration of the desired ions is accomplished by forming a complex of the desired ions with an EGTA ligand bound membrane. The separation is effected in a separation device, such as a membrane cartridge, through which the source solution is flowed. This process enables the desired ions to complex with the EGTA ligand attached to the membrane. The metal ion and the EGTA ligand are then decoupled by flowing a receiving liquid through the separation device (in much smaller volume than the volume of source solution passed through the column) to remove and concentrate the desired ions in the receiving liquid solution. The receiving liquid or recovery solution forms a stronger complex with the desired ions than does the EGTA, or alternatively, temporarily forms a stronger interaction with the EGTA ligand than do the desired metal ions, and thus, the desired metal ions are quantitatively stripped from the ligand in a concentrated form in the receiving solution. The recovery of desired ions from the receiving liquid may be accomplished by various methods commonly known in the art.
The present invention provides a composition and method for the removal and/or separation of particular divalent metal ions including Ca2+ and/or Cd2+ present in low concentrations from a solution utilizing an EGTA ligand attached to a membrane. The present invention is particularly adaptable to the removal of (a) Ca2+ from source solutions containing Mg2+ and/or other ions, and (b) Cd2+ from source solutions containing Zn2+ and/or other ions. The invention may also be carried out in moderately acidic solutions, e.g., solutions with a pH greater than about 3.0. Furthermore, the desired selectivity and interaction strength is unexpectedly high despite the fact that one of the four carboxylic acid groups of the ligand is used to attach the ligand to the membrane support.
The present invention requires that the ligand be covalently bonded to a membrane which acts as a support to the EGTA ligand. Specifically, the composition of the present invention comprises an EGTA ligand that is covalently bonded to a membrane as represented Ad Formula 1, as follows:
Mxe2x80x94Bxe2x80x94L xe2x80x83xe2x80x83Formula 1
where M is any membrane or composite membrane derivatized to have a hydrophilic surface and which contains polar functional groups, L is an EGTA ligand and B is the covalent linkage joining the ligand (L) to the membrane (M) surface. Typically, the EGTA ligand (L) is joined to the membrane surface by the reaction of ligand precursor XL where X is a functional group which reacts with an activated polar group on the membrane surface, thereby forming covalent linkage B. Representative of B linkages are members selected from the group consisting of amide (xe2x80x94NHC(O)xe2x80x94, ester (xe2x80x94C(O)Cxe2x80x94), thioester (xe2x80x94C(O)Sxe2x80x94), carbonyl (xe2x80x94C(O)xe2x80x94), ether (xe2x80x94Oxe2x80x94), thioether (xe2x80x94Sxe2x80x94), sulfonate (xe2x80x94S(O)2Oxe2x80x94) and sulfonamide (xe2x80x94SO2NHxe2x80x94), though amide bonds are often preferred.
More specifically, the membrane (M) may be inherently hydrophilic, partially hydrophilic or a composite comprising a porous polymer membrane substrate having an insoluble cross-linked hydrophilic coating deposited thereon. Membranes that are inherently hydrophilic or partially hydrophilic and contain moieties appropriate for forming covalent bonds with the ligand (L) have particular utility. Such membranes include polyamides such as nylon, and cellulosic materials such as cellulose, regenerated cellulose, cellulose acetate and nitrocellulose. If the membrane used does not contain reactive groups, it may be modified or derivatized appropriately.
Composite membranes are also preferred. A composite membrane comprises a porous polymer or copolymer membrane core and an insoluble coating deposited thereon. The substrate and the coating may be joined by crosslinking, grafting or by other known procedures. Representative suitable polymers forming the membrane core substrate include fluorinated polymers including poly (tetrafluoroethylene) (xe2x80x9cTEFLONxe2x80x9d), polyvinylidene fluoride (PVDF), and the like; polyolefins such as polyethylene, ultra-high molecular weight polyethylene (UPE), polypropylene, polymethylpentene, and the like; polystyrene or substituted polystyrenes; polysulfones such as polysulfone, polyethersulfone, and the like; polyesters including polyethylene terephthalate, polybutylene terephthalate, and the like; polyacrylates and polycarbonates; polyethers such as perfluorinated polyethers; and vinyl polymers such as polyvinyl chloride and polyacrylonitriles. Copolymers can also be used for forming the polymer membrane, such as copolymers of butadiene and styrene, fluorinated ethylene-propylene copolymer, ethylene-chlorotrifluoroethylene copolymer, and the like. The preferred membrane is a hydrophilic ultrahigh molecular weight polyethylene (UPE) containing carboxylic groups, such as those described in U.S. Pat. Nos. 4,618,533, 5,618,433 and 5,547,760.
The membrane is selected to yield both selected bulk properties and selected surface properties. For naturally hydrophilic membranes, the selected bulk and surface properties will be provided by the polymer of which the membrane is formed. For composite membranes, the selected bulk properties will be provided by the membrane substrate and the selected surface properties will be provided by the coating.
A composite membrane is formed by depositing a monomer directly on the surface of the substrate, including the inner surfaces of the pores, by in situ deposition of the cross-linked monomer. The desired deposition of the cross-linked monomer onto the porous substrate is effected as a direct coating and does not require or utilize an intermediate binding chemical moiety. Any monomer for the coating polymer can be used so long as it is capable of being polymerized by free radical polymerization and can be crosslinked. The only requirements of the polymerized monomer is that it is capable of coating the entire surface of the porous membrane, that it provide the surface with ligand-reactive functional groups and that it be sufficiently hydrophilic to allow for efficient use of the ligand to be attached. Generally, the porous substrate has an average pore size between about 0.001 and 10 xcexcm and more usually, between about 0.1 and 5.0 xcexcm. The composite membrane is formed by any suitable method, such as is disclosed in U.S. Pat. No. 4,618,533, which is hereby incorporated by reference in their entirety. Briefly, this procedure involves washing the porous membrane substrate with a suitable solvent for wetting the entire surface of the substrate. The substrate is then bathed in a mixture of the free radical polymerizable monomer, a polymerization initiator, and a cross-linking agent in a solvent under conditions to effect free radical polymerization of the monomer and coating of the porous substrate with the cross-linked polymer. The surface of the coated polymer membrane contains hydrophilic or polar-substituents that can be activated to react with and covalently bond the ligands to the membrane surface.
The composite membranes prepared according to U.S. Pat. No. 4,618,533 can contain carboxylic acid moieties on the surface. Other suitable moieties could include hydroxyl, sulfonic acid, epoxy, primary amine and derivatized benzyl groups such as polymers referenced above.
Preparation of a composite membrane by a precipitated crystal technique involves, briefly, washing the porous membrane substrate with a suitable solvent for wetting the entire surface of the substrate. The substrate is then bathed in a solution containing the compound which is to be precipitated. This solution is then removed and the membrane substrate is treated with a compound that precipitates and fixes the crystals to the substrate. The membrane is washed and dried before use.
With composite membranes, the core membrane material is not thought to affect that performance of the derivatized membrane and it is limited in composition only by its ability to be coated, or have deposited on its surface, an insoluble polymer layer that contains the appropriate reactive group. This provides a hydrophilic layer which interacts well with water or other aqueous solutions. The end result is that when the ligand is attached to the surface of either a hydrophilic membrane or a composite membrane having a hydrophilic surface, the basic characteristics of any given ligand molecule are not changed by the process of attaching it to the surface or by the nature of the surface itself.
Formulas similar to the Mxe2x80x94Bxe2x80x94 portion of Formula 1 have been previously described in U.S. Pat. No. 5,547,760 and U.S. Pat. No. 5,618,433, which are incorporated herein by reference. These membranes and attachment mechanisms are instructive to one skilled in the art in practicing the present invention. However it is to be emphasized that the present invention does not reside in the discovery of the Mxe2x80x94Bxe2x80x94 portion of Formula 1. Rather, it is the discovery of the advantageous ion binding capabilities of the EGTA ligand (L) when attached to a membrane (M) via a covalent linkage (B).
EGTA ligands covalently bonded to membranes as shown in Formula 1 are characterized by high selectivity for and removal and separation of desired divalent metal ions, such as Ca2+ and/or Cd2+, present at from very low to very high concentrations in various source solutions, including source solutions that contain other divalent ions. The source solutions for which the present invention is particularly useful would contain the desired ions and other ions present in greater concentrations which one does not desire to remove. Such other ions can include H+, Mg2+ and Zn2+.
The present invention also provides methods for removing desired divalent metal ions from solutions that can contain other metal ions present in much higher concentrations by the use of such EGTA ligand bonded membranes disclosed herein. These methods may be carried out in any manner that provides for bringing the ions to be removed from a solution into contact with the EGTA ligands affixed to the membrane. Contact is preferably made in a contacting device comprising a housing, e.g., cartridge, containing the composition of matter of the invention by causing solution containing desired ions to flow through the cartridge and thus come in contact with the composition of the invention. Preferably the membrane configuration is a pleated membrane, although other membrane configurations, such as flat sheet, stacked disk or hollow fibers may be used. However, various contact apparatus may be used instead of a cartridge such as but not limited to a cassette, syringe, unit, canister, multi-well plate or filter holder. The process of selectively removing and concentrating the desired ions is characterized by the ability to quantitatively complex from a larger volume of solution the desired ions, even when they are present at very low concentrations.
The desired ions are recovered from the ligands bound to the membranes by flowing through a small volume of a receiving phase which contains a solubilizing reagent. The solubilizing reagent need not be selective, but merely be capable of forming a stronger complex with the desired divalent metal ion than does the EGTA ligand. Alternatively, the solubilizing reagent may contain ions that temporarily form a stronger interaction with the EGTA ligand than does the desired metal ions. This causes the desired metal ion to be quantitatively stripped from the ligand bound membranes in concentrated form and transferred to the receiving solution. The recovery of the desired metal ion from such a receiving solution is readily accomplished by evaporation, precipitation, or by other known methods.
There are several applications for which one may wish to remove/separate the aforementioned divalent metal ions from solution. An example of such an application is the separation of ppb levels of Cd2+ or other divalent metals from concentrated Cu, Ni, Zn, Ag, Pb and/or Hg contained in process streams and industrial effluents. This is true both because of the unsatisfactory technologies presently available or because a more economical method is desired. For example, Cd2+ is often present at low concentrations in solutions containing other divalent ions at much greater concentrations. Therefore, the removal of Cd2+ from such solutions would be important environmentally because of impurity concerns. Additionally, quantitation and/or removal of Cd2+ or Ca2+ ions from other non-desirable ions may also be employed. Specifically, this removal process may be implemented when the desired ions (Cd2+ and/or Ca2+) are present at from very low levels (ppb) to very high levels (g/l), even in the presence of concentrated Cu, Ni, Zn, Ag, Pb and/or Hg process streams.
All of these functions and others may be accomplished by use of the compositions and methods of the present invention. The invention will now be illustrated by the following examples which are not intended to be limiting in any way. All references cited are incorporated herein by reference in their entirety.